I often get PSD assets that have layers with blending modes that end up blending them into a background layer. I'm trying to figure out of there is any way to cleanly extract these layers from the background, ending up with a transparent background, but keeping the color effects that were created by blending onto, say, a blue background?

Hopefully that's clear...

A very simplified example:

blue background

take a white brush and paint a line on a higher layer

set this layer to Overlay

You end up with a light-blue streak with blurred edges.

I ultimately want an image that has this same light-blue color but blends to transparent instead of blue.

Obviously real-life examples are much more complicated, but hopefully this explains what I'm trying to do :)

Duplicate layer to move the squiggle into a layer with transparent background

Or use select color range, select the background, quick mask, then delete to remove background.

The other thing you could try is duplicate your background layer, move it to the top, and set blend mode to subtract or difference. You'll get a black background with the "white" squiggle, but the squiggle will be a bit darker too.

If I understand the question correctly, you want the resulting color appearance and the shape with that color appearance to become a layer, with the (previously blended) color now a full "native" color and the remainder of the layer transparent.

Here's how I would tackle it, given this as a starting point (your example):

First create a new layer that duplicates the color appearance of the current composite. You do this by targeting the current top layer, then using Ctl-Alt-Shift-E (Cmd-Opt-Shift-E) to create a new layer. This basically creates a new layer and applies "Merge Visible" into the new layer.

Ctl/Cmd-click on the layer thumbnail of your blended layer to create a selection. (Don't target the layer, just Ctl/Cmd-click the thumbnail in the layers panel.)

The new solid layer should still be targeted in the layers panel. Press Ctl/Cmd-J to copy that portion of the solid layer onto its own layer.

I knew my simplified example would bite me :) What if the top layer is 100% opaque but only parts of it visible due to a blending mode? E.g. a white square with a red squiggle through it, blended so the white doesn't show up
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cwolvesSep 22 '11 at 20:12

Also, doesn't quite work. If you take this new image and put the same color blue behind it there's an odd halo
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cwolvesSep 22 '11 at 20:14

I'm having trouble with the exact scenario you're describing. Is it the "blend-if" kind of blending or blend modes? Can you comp a before/after of a simplified example of the desired effect. BTW: The solution I proposed probably shows the halo because you used a soft brush and the edge feathering interacts with the bottom layer.
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Steve RossSep 22 '11 at 20:34